Electrode material, electrode, and method for hydrogen chloride electrolysis

ABSTRACT

An electrode material, an electrode and a process for hydrogen chloride electrolysis based on platinum metal as catalyst, in which the electrode material has a nanosize mixture of platinum particles and silver particles, is described.

The invention relates to an electrode material based on platinum metalas catalyst, an electrode composed thereof and a process for hydrogenchloride electrolysis.

Starting from polyvinyl chloride (PVC) via foams through to medicamentsand crop protection agents, chlorine chemistry contributes up to 60% ofthe sales of the German chemical industry. The high reactivity andselectivity of chlorine as reactant is usually exploited here, and thechlorine is obtained again as by-product or coproduct in the form ofhydrochloric acid. Apart from other uses of hydrochloric acid(marketing, use in other processes), it can be electricallyredissociated to recycle chlorine. In a further development of theclassical electrolysis process, electrolyzers which avoid evolution ofhydrogen on the cathode side and instead have oxygen-consuming cathodesat which oxygen-containing gases can be reduced are increasingly usednowadays for hydrogen chloride electrolysis. In this way, up to 30% ofthe required energy can be saved by reducing the electrolysis voltage.Although platinum is in principle outstanding by having the highestactivity and selectivity for the reduction of oxygen, preference isgiven to using supported rhodium sulfide catalysts. The reason for thisis the highly corrosive conditions under which the HCl electrolysistakes place, which lead to deactivation and dissolution of platinum. Inview of the high raw materials price for rhodium and the lower activitycompared to platinum, an improved catalyst based on platinum would be ofgreat economic importance in the context of increasing energyconsumption and increasingly scarce resources.

In the membrane electrolysis of hydrogen chloride, the electrodematerial is exposed to relatively harsh conditions. Thus, it has towithstand the corrosive chlorine-containing solution which cannot becompletely held back from the cathode side by the polymer membrane andin the case of platinum as catalyst is reduced to chlorine over theplatinum. In the case of platinum, the effect of chloride ions ascatalyst poison has been adequately studied. Particularly in the case ofa shutdown of the electrolysis plant (planned or unplanned due tobottlenecks in power supply), a sudden increase in potential at theplatinum in the presence of dissolved chlorine and chloride ions canlead to a substantial loss of catalyst material due to dissolution ofthe platinum in the form of hexachloroplatinic acid and to deactivationof the remaining catalyst material (see J. R. Giallombardo, D. Czerwiec,E. S. De Castro, C. K. Shaikh, F. Gestermann, H.-D. Pinter, G. Speer, R.J. Allen. “Process for the electrolysis of technical-grade hydrochloricacid contaminated with organic substances using oxygen-consumingcathodes”, U.S. Pat. No. 6,402,930 and A. P. Yadav, A. Nishikata and T.Tsuru. Electrochim. Acta (2007), 52 [26], pages 7444-7452). However,regardless of any shutdown of the plant, a rise in the overvoltage forthe reduction of oxygen over polycrystalline platinum catalysts can beobserved in the presence of chloride ions (see T. J. Schmidt, U. A.Paulus, H. A. Gasteiger and R. J. Behm. J. Electroanal. Chem. (2001),508 [1-2], pages 41-47).

To solve the problem of the low stability of the platinum catalyst forHCl electrolysis, novel, supported rhodium sulfide-based (Rh_(x)S_(y))catalysts for HCl electrolysis which are virtually insensitive to manyorganic and inorganic compounds and do not suffer a loss of catalyst inthe event of a shutdown of the electrolysis plant have been developed.An electrode material based on rhodium sulfide catalysts, an electrodeand a corresponding HCl electrolysis process are subject matter of theinternational application WO 2002 018 675 A2. The electrode materialdescribed here is, owing to its greater chemical stability, used insteadof electrode material based on platinum catalyst. WO 2002 018 675 A2explicitly describes this advantage of the rhodium sulfide catalyst.However, its deficiency is the lower catalytic activity compared toplatinum. In view of the comparatively high commercial price of rhodiumand the lower catalytic activity compared to platinum, there continuesto be a need for new and improved catalyst materials for electrodes, inparticular cathodes of HCl electrolyzers. The successful furtheroptimization would, moreover, be important not only for HCl electrolysisbut also for other electrolysis processes such as chloralkalielectrolysis. Even small improvements can achieve large effects inenergy saving. Thus, each millivolt saved in the cell voltage in thechloralkali electrolysis could achieve an annual worldwide saving of 32million kWh.

It is therefore an object of the present invention to provide novel andimproved catalyst materials for HCl electrolysis, which should have ahigher activity than the rhodium sulfide catalyst used at present whilemaintaining an equally high chemical stability under the conditions ofthe industrial HCl electrolysis.

A further object of the invention is to provide an electrode materialand an electrode based thereon which avoids the disadvantages of theknown electrodes and has a comparatively longer operating life in theHCl electrolysis.

The object is achieved by using an electrode material which is based ona mixture of nanoparticulate platinum metal and silver metal.

The invention provides an electrode material for hydrogen chlorideelectrolysis which is based on platinum metal as catalyst, characterizedin that the electrode material has a nanosize mixture of platinumparticles and silver particles, where platinum and silver have aparticle diameter of essentially not more than 1 μm, preferably not morethan 0.5 μm, particularly preferably not more than 0.1 μm.

The novel electrode material of the invention can be used either insupported form on a conductive inert support or in unsupported fowl.

The novel electrode material does not require any activation step beforeuse and retains its full electrode catalytic activity in respect of thereduction of oxygen even in the presence of chloride ions. Furthermore,the novel electrode material is not dissolved by the complexing actionof mixtures of aqueous hydrochloric acid and chlorine gas, so that nospecific precautionary measures are necessary when shutting down thehydrochloric acid electrolyzers in which the electrode material is used.

To produce the novel gas diffusion electrodes, the novel electrodematerial is preferably applied to at least one side of a conductivesheet-like textile structure. The novel electrode material can be usedeither alone or together with a binder mixed with a conductive supportmaterial or supported on a conductive support material and combined witha binder. The binder can be hydrophobic or hydrophilic and the mixturecan be applied to one or both sides of the sheet-like structure.

Preferred binders are fluoropolymers such as polytetrafluoroethylene(PTFE, commercially available, inter alia, under the name Teflon® (fromDuPont)), polyvinylidene difluoride (PVDF), polymeric perfluorosulfonicacids (PFSA, obtainable, inter alia, under the name Nafion® (fromDuPont)) or other proton-conducting ionomers known to those skilled inthe art.

Electrode structures or base materials containing gas diffusion layersas are known from EP 0931857 and U.S. Pat. No. 4,293,396 and can beobtained, inter alia, under the name ELAT® (from BASF Fuel Cell Inc.)can typically be used.

The sheet-like structure can be a woven fabric or a nonwoven made ofelectrically conductive material or consist of a carbon cloth, carbonpaper or any conductive metal mesh.

Examples of preferred support materials, in particular support materialshaving a large surface area, encompass graphite, various forms ofcarbon, in particular carbon nanotubes, and other finely dividedsupports, with carbon black being particularly preferred.

Such sheet-like structures coated with the novel electrode material canbe used as gas diffusion cathodes which achieve a cell voltage and longlife which have hitherto not been able to be achieved under conventionaloperating conditions. This applies particularly to the use of theelectrode material in highly aggressive environments as occur in theelectrolysis of hydrochloric acid as by-product.

Preference is given to an electrode material in which the weight ratioof platinum to silver is from 10:90 to 90:10, preferably from 30:70 to70:30, particularly preferably from 40:60 to 60:40.

Further preference is given to an electrode material which ischaracterized in that the material additionally has particles composedof alloys of platinum and silver.

An advantageous preferred electrode material has platinum particles andsilver particles and optionally alloy particles which have,independently of one another, an average particle diameter in the rangefrom 1 nm to 100 nm, preferably from 2 nm to 50 nm and particularlypreferably from 3 to 25 nm.

In particular, the platinum and silver particles can form agglomerateshaving an average agglomerate diameter of less than 100 μm, preferablyless than 10 μm.

A particularly preferred electrode material is characterized in that theplatinum and silver particles are obtained by simultaneouselectrodeposition of platinum and silver, in particular byelectrodeposition using a pulsed voltage, from platinum and silver saltsolutions or melts, in particular from aqueous platinum and silver saltsolutions, onto an electrically conductive support material.

The electrodeposition using a pulsed voltage is preferably carried outat an open-circuit voltage of from 0.4 to 0.8 V measured relative to asilver-silver chloride reference electrode in 3 molar potassium chloridesolution, using voltage pulses in the range from −0.4 to −0.8 V and apulse length in the range from 5 to 100 ms.

The invention further provides a chlorine-resistant electrode forelectrochemical processes which has an electrode material based on amixture of platinum and silver and can be installed as cathode inhydrogen chloride electrolysis.

A preferred chlorine-resistant electrode has the novel electrodematerial.

In one embodiment, the electrode is particularly preferably anoxygen-consuming cathode.

In the embodiment as oxygen-consuming cathode, the electrode isconfigured as gas diffusion electrode having an electrically conductivesheet-like textile structure as support, in particular a mesh, which isprovided on at least one side with a catalyst which comprises theelectrode material and optionally additionally comprises at least onebinder containing fluorine compounds incorporated therein.

Preference is given to a gas diffusion electrode in which the conductivesheet-like structure is provided on one or both sides with a coatingwhich comprises at least one fluoropolymer and at least one electricallyconductive carbon material and is additionally coated on only one sidewith a mixture of the catalyst and at least one fluoropolymer.

In another embodiment, the electrode is particularly preferably ahydrogen-evolving cathode.

In the embodiment as hydrogen-evolving cathode, the electrode is, inparticular, a graphite electrode in which the electrode material isapplied as catalytically active coating to a graphite support.

The invention also provides a membrane-electrode assembly whichcomprises an ion-exchange membrane which is provided on at least oneside with a catalyst comprising the electrode material of the invention.

The invention further provides for the use of the electrode of theinvention or the membrane-electrode assembly of the invention for theelectroreduction of oxygen.

The invention further provides an electrochemical cell having at leastan anode chamber containing an anode and a cathode chamber containing acathode, which are separated from one another by a separator, where thecathode is an electrode according to the invention.

The invention also provides an electrochemical cell having at least ananode chamber containing an anode and a cathode chamber containing acathode, which are separated from one another by a separator, where theseparator is configured as a membrane-electrode assembly according tothe invention.

In a preferred embodiment of the electrochemical cell, the separator isan ion-exchange membrane or a diaphragm.

Particular preference is given to embodiments of the abovementionedtypes of electrochemical cells which are characterized in that the anodechamber can be supplied with aqueous hydrochloric acid and the cathodechamber can be supplied with an oxygen-containing gas or with aqueoushydrochloric acid.

The invention further provides a process for the electrolysis of anaqueous hydrochloric acid solution to form chlorine, characterized inthat aqueous hydrochloric acid is fed into the anode chamber and anoxygen-containing gas is fed into the cathode chamber in a novelelectrochemical cell of the abovementioned types while the cell issupplied with an electric direct current.

The invention is illustrated below with the aid of FIG. 1 and theexamples which, however, do not restrict the invention.

In the figures:

FIG. 1 a+b show scanning electron micrographs of the glassy carbonsurface after electrodeposition of silver and platinum,

FIG. 2 shows an energy-dispersive X-ray spectrum of the glassy carbonelectrode coated with platinum-silver nanoparticles,

FIG. 3 schematically shows the flow cell for testing the stability ofthe glassy carbon electrode coated with platinum-silver nanoparticles,

FIG. 4 shows chronoamperograms of a glassy carbon electrode coated withplatinum-silver nanoparticles and a platinum-coated glassy carbonelectrode,

FIG. 5 a+b shows the stability of the platinum-silver-coated glassycarbon electrode compared to the platinum-coated glassy carbonelectrode.

EXAMPLES Example 1 Production of the Pt—Ag Electrodes of the Invention

The Pt—Ag electrodes were produced by simultaneous electrodeposition ofplatinum and silver from a 10 millimolar ethylenediamine solution (pH11) which was 3 millimolar in hexachloroplatinic acid and 3 millimolarin silver nitrate onto a glassy carbon electrode (diameter 3 mm). Priorcleaning of the glassy carbon electrode was carried out by mechanicalpolishing using various Al₂O₃ suspensions (average particle diameter: 1μm, 0.3 μm and 0.05 μm) on a polishing felt.

Electrodeposition was carried out in a three-electrode system underpotentiostatic control at room temperature in a single-compartment cellfrom 1 ml of solution volume. Apart from the glassy carbon workingelectrode, a platinum wire was used as counterelectrode (CE) and asilver helix was used as reference electrode (RE). The pulse profileshown in Table 1 was selected for the deposition.

TABLE 1 Pulse profile for the simultaneous electrodeposition of platinumand silver Potential [V] vs. Ag/AgCl (3M KCl) Time [s] E1 +0.60 5 E2−0.25 0.005 E3 (Pt—Ag) −0.65 25

The scanning electron micrographs (SEM) in FIGS. 1 a and 1 b show thatthe pulse profile selected leads to deposition of nanoparticles on theglassy carbon surface.

The platinum-silver content of nanoparticles can be found to be 50:50 bymeans of energy-dispersive X-ray spectroscopy (EDX) (see spectrum inFIG. 2).

Example 2

As comparative material, a platinum-modified electrode was produced byelectrodeposition of platinum onto a glassy carbon electrode (diameter:3 mm). The deposition of platinum was carried out by a method analogousto the deposition of the platinum-silver nanoparticles in Example 1 froma 10 millimolar ethylenediamine solution (pH 11) which was 3 millimolarin hexachloroplatinic acid, at a potential E3 of −0.75 V (25 s).

Example 3 Stability Test on the Pt—Ag Electrode and the Pt Electrode

The glassy carbon electrode coated with platinum-silver nanoparticlesfrom Example 1 was simultaneously tested in comparison with the glassycarbon electrode coated only with platinum from Example 2 to determineits stability toward chlorine and chloride ions in an electrochemicalflow cell (see FIG. 3).

FIG. 3 schematically shows the flow cell for the stability test. In theelectrolysis cell (left-hand cell in FIG. 3) there are two platinum diskauxiliary electrodes (Ø1 mm, spacing 4 mm) which are located oppositeone another and at which chloride was oxidized to chlorine during theentire time of the experiment. This was achieved by application of anexternal voltage of 1.5 V, which was provided by a simple laboratoryvoltage source, between the two auxiliary electrodes. The auxiliaryelectrodes were polished in a manner analogous to the glassy carbonelectrodes before each experiment. The measurement of the stability ofthe glassy carbon electrode coated with platinum-silver nanoparticlesand of the glassy carbon electrode coated only with platinum was carriedout chronoamperometrically in the electrolysis cell 2 (right-hand cellin FIG. 3) at a potential of −0.15 V vs. Ag/AgCl (3 molar KCl) at whichoxygen is reduced at the working electrodes to be examined (WE 1 and WE2), at room temperature. The actual measurement cell (electrolysis cell2) has a volume of about 200 μl, and the catalyst-coated electrodes havea spacing of 4 mm and are opposite one another. As counterelectrode(CE), use is made of a stainless steel capillary through which thesolution flows out from the cell, and an Ag/AgCl (3 molar KCl) electrodeserved as reference electrode (RE). Aqueous 0.4 molar hydrochloric acidis pumped through the two cells at a pumping rate of 28 ml/h. This isloaded with chlorine in electrolysis cell 1 and then goes intoelectrolysis cell 2 in which the actual stability test is carried out.

The application of the potentials to the two working electrodes waseffected by means of an 8-fold potentiostat from CH Instruments. Tosimulate the shutdown of an industrial HCl electrolysis cell, the cellwas operated for 30 minutes and the potential was then switched off bymeans of a relay for 1 minute. The procedure was repeated ten times withthe application of the oxygen reduction potential being shortened to 12minutes. The currents which flowed were recorded for both electrodes tobe examined during the entire time of the experiment. Thechronoamperogram obtained is shown in FIG. 4.

The evaluation of the chronoamperograms is shown in FIGS. 5 a and 5 b.

FIG. 5 a) (at left) and b) (at right) show the stability of theplatinum-silver-coated glassy carbon electrode compared to theplatinum-coated glassy carbon electrode; the currents indicated in FIG.5 a) were recorded at the end of the 12 min of the oxygen reductionphase shortly before the electrolysis cell was switched off again. FIG.5 b) shows the measured oxygen reduction currents normalized to therespective initial reduction current (before the 1st switching-off).

After the first switching-off, the absolute value of the oxygenreduction current for the platinum-silver-coated glassy carbon electrodewas already greater than that for the platinum-coated electrode. With anincreasing number of switching-off operations, the absolute value of thereduction current for the electrode coated only with platinum decreasedmore and more, while in the case of the glassy carbon electrode coatedwith platinum-silver it decreased only slightly to a then constant valueof over 90% of the initial reduction current. The activity of the glassycarbon electrode coated with platinum-silver thus proved to be stable tothe switching-off operations while the electrode coated with platinumwas shown to be unstable against the switching-off operations.

1.-21. (canceled)
 22. An electrode material for hydrogen chlorideelectrolysis which is based on platinum metal as catalyst, wherein theelectrode material has a nanosize mixture of platinum particles andsilver particles, wherein the platinum and silver particles have aparticle diameter of essentially not more than 1 μm.
 23. The electrodematerial as claimed in claim 22, wherein the weight ratio of platinum tosilver is from 10:90 to 90:10.
 24. The electrode material as claimed inclaim 22, wherein the electrode material additionally comprisesparticles comprising alloys of platinum and silver.
 25. The electrodematerial as claimed claim 22, wherein the platinum particles, silverparticles, and alloy particles have, independently of one another, anaverage particle diameter in the range of from 1 nm to 100 nm.
 26. Theelectrode material as claimed in claim 22, wherein the platinum andsilver particles form agglomerates having an average agglomeratediameter of less than 100 μm.
 27. The electrode material as claimed inclaim 22, wherein the platinum and silver particles are obtained bysimultaneous electrodeposition of platinum and silver from platinum andsilver salt solutions or melts onto an electrically conductive supportmaterial.
 28. The electrode material as claimed in claim 27, wherein theelectrodeposition is carried out using a pulsed voltage and is carriedout at an open-circuit voltage of from 0.4 to 0.8 V measured relative toa silver-silver chloride reference electrode in 3 molar potassiumchloride solution, using voltage pulses in the range from −0.4 to −0.8 Vand a pulse length in the range from 5 to 100 ms.
 29. Achlorine-resistant electrode for electrochemical processes whichcomprises an electrode material based on a mixture of platinum andsilver, wherein the electrode can be installed as a cathode in hydrogenchloride electrolysis.
 30. A chlorine-resistant electrode forelectrochemical processes which comprises the electrode material asclaimed in claim
 22. 31. The electrode as claimed in claim 29, whereinthe electrode is an oxygen-consuming cathode.
 32. The electrode asclaimed in claim 31, wherein the electrode is configured as a gasdiffusion electrode having an electrically conductive sheet-like textilestructure as support, wherein at least one side of the electrodecomprises a catalyst which comprises the electrode material andoptionally additionally comprises at least one binder comprisingfluorine compounds incorporated therein.
 33. The electrode as claimed inclaim 32, wherein the conductive sheet-like structure is provided on oneor both sides with a coating which comprises at least one fluoropolymerand at least one electrically conductive carbon material and isadditionally coated on only one side with a mixture of the catalyst andat least one fluoropolymer.
 34. The electrode as claimed in claim 29,wherein the electrode is a hydrogen-evolving cathode.
 35. The electrodeas claimed in claim 34, wherein the electrode is a graphite electrode inwhich the electrode material is applied as a catalytically activecoating to a graphite support.
 36. A membrane-electrode assembly whichcomprises an ion-exchange membrane which is provided on at least oneside with a catalyst comprising the electrode material as claimed inclaim
 22. 37. An electrochemical cell comprising at least an anodechamber comprising an anode and a cathode chamber comprising a cathode,wherein the anode and the cathode are separated from one another by aseparator, and wherein the cathode is the electrode as claimed in claim29.
 38. The electrochemical cell as claimed in claim 37, wherein theseparator is an ion-exchange membrane or a diaphragm.
 39. Anelectrochemical cell comprising at least an anode chamber comprising ananode and a cathode chamber comprising a cathode, wherein the anode andthe cathode are separated from one another by a separator, and whereinthe separator is configured as the membrane-electrode assembly asclaimed in claim
 36. 40. The electrochemical cell as claimed in claim37, wherein the anode chamber can be supplied with aqueous hydrochloricacid and the cathode chamber can be supplied with an oxygen-containinggas or with an aqueous hydrochloric acid.
 41. A process for theelectrolysis of an aqueous hydrochloric acid solution to form chlorine,wherein aqueous hydrochloric acid is fed into the anode chamber and anoxygen-containing gas is fed into the cathode chamber in anelectrochemical cell as claimed in claim 37 while the electrochemicalcell is supplied with an electric direct current.